Phosphorus transport and retention in a channel draining an urban, tropical catchment with informal settlements

P M Nyenje,S Uhlenbrook,R Kulabako,L M G Meijer,J W Foppen

doi:10.5194/hess-18-1009-2014

Abstract

Abstract. Urban catchments in sub-Saharan Africa (SSA) are increasingly becoming a major source of phosphorus (P) to downstream ecosystems. This is primarily due to large inputs of untreated wastewater to urban drainage channels, especially in informal settlements (or slums). However, the processes governing the fate of P in these catchments are largely unknown. In this study, these processes are investigated. During high runoff events and a period of base flow, we collected hourly water samples (over 24 h) from a primary channel draining a 28 km2 slum-dominated catchment in Kampala, Uganda, and from a tertiary channel draining one of the contributing slum areas (0.54 km2). The samples were analysed for orthophosphate (PO4-P), particulate P (PP), total P (TP), suspended solids (SS) and hydrochemistry. We also collected channel bed and suspended sediments to determine their geo-available metals, sorption characteristics and the dominant phosphorus forms. Our results showed that the catchment exported high fluxes of P (0.3 kg km2 d−1 for PO4-P and 0.95 for TP), which were several orders of magnitude higher than values normally reported in literature. A large proportion of P exported was particulate (56% of TP) and we inferred that most of it was retained along the channel bed. The retained sediment P was predominantly inorganic (> 63% of total sediment P) and consisted of mostly Ca and Fe-bound P, which were present in almost equal proportions. Ca-bound sediment P was attributed to the adsorption of P to calcite because surface water was near saturation with respect to calcite in all the events sampled. Fe-bound sediment P was attributed to the adsorption of P to iron oxides in suspended sediment during runoff events given that surface water was undersaturated with respect to iron phosphates. We also found that the bed sediments were P-saturated and showed a tendency to release P by mineralisation and desorption. During rain events, there was a flushing of PP which we attributed to the resuspension of P-rich bed sediment that accumulated in the channel during low flows. However, first-flush effects were not observed. Our findings provide useful insights into the processes governing the fate and transport of P in urban slum catchments in SSA.

Highlights

Phosphorus (P) derived from urban catchments in subSaharan Africa (SSA) is increasingly becoming a major cause of eutrophication of urban fresh water bodies (Nhapi et al, 2002; Nyenje et al, 2010)
particulate P (PP) was the dominant form of P in the primary channel during base flows whereas in the tertiary channel PO4-P was dominant
The corresponding P fluxes of 0.3 kg km2 d−1 for PO4-P and 0.95 kg km2 d−1 for total P (TP) were of several orders of magnitude higher than values normally reported in published literature for other catchments

Summary

Introduction

Phosphorus (P) derived from urban catchments in subSaharan Africa (SSA) is increasingly becoming a major cause of eutrophication of urban fresh water bodies (Nhapi et al, 2002; Nyenje et al, 2010) This is primarily attributed to the increasing release of untreated or partially treated wastewater into the environment especially in the informal settlements or slums. Nyenje et al.: Phosphorus transport and retention in a channel draining an urban, tropical catchment (nitrogen, N, and phosphorus, P) to downstream fresh water bodies (NWSC, 2008; Kulabako et al, 2010; Isunju et al, 2011; Bere, 2007; Nhapi and Tirivarombo, 2004; Nhapi et al, 2002; Foppen and Kansiime, 2009; Katukiza et al, 2010).

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